1. Field of the Invention
The present invention relates to a method of manufacturing a molded multilayer article and an apparatus therefor. More specifically, the present invention relates to a method and an apparatus which is capable of efficiently producing molded multilayer article in a wide variety of sizes and shapes by combining a process of laminating a plurality of extruded molten polymer layers and a process of compression molding.
2. Description of the Related Art
Known processes of manufacturing a monolayer article of a thermoplastic polymer adapt a combination of an injection molding process and a press forming process, or a combination of an extrusion molding process and a vacuum forming process for shaping a film or sheet into a article with a relatively broad width and a complicated three-dimensional shape.
A sheet is formed by extruding a molten polymer through an extrusion die, and successively the sheet is fed to molds for a compression molding process. Therefore, the extrusion die and the associated parts must be moved toward the molds. Various devices adapted for moving an extrusion die along a predetermined path over the bottom half mold of an open mold have been disclosed, for example, in JPB No. 17931/1982 and JPA No. 137814/1988.
A stampable sheet molding process applied to processes for producing that kind of sheet products is one of the known processes. In the stampable sheet molding process, a sheet formed by extrusion is cut into a workpiece with a predetermined size, and the workpiece is softened by heating and delivered to final molding process on a compression molding machine. Such a processing method comprises steps of extrusion, forming, cutting process, heating process, and compression molding. However, additional electric power consumption is increased for heating process and limited varieties of shape of the products to be processed by the compression molding is one of the drawbacks.
The extrusion die employed in those known techniques has a die slot opening of a fixed dimension. Therefore the extrusion die can be used only for extruding a sheet of a predetermined fixed width and is not applicable to extrusion of a sheet with varying width and thickness.
An apparatus disclosed in JPB No. 25689/1989 is adapted for molding finished sheet products with varying width by extruding a sheet with varying width, which is feed to compression molding machines.
This apparatus is capable of changing the sectional shape of a sheet which is extruded through a die disposed opposite to the compression molding machine according to the shape of an finished product.
The foregoing prior art techniques relate to monolayer articles. One kind of molded articles has a structure of laminated layers consisting of sheets or films in different in strength, hardness, color or such. For example, one of molded multilayers articles consists of a base layer, a mid-layer and a skin layer.
Prior art methods of manufacturing molded multilayer articles of the abovementioned kind and apparatus therefor are disclosed in JPB No. 2491/1993 and JPA No.24128/1993. In these prior art, a multilayer sheet is formed by co-extruding molten polymers through a multilayer T (slot) die, and the multilayers sheet are fed to molds of a compression molding machine for the subsequent compression molding process.
Since the multilayer sheet is formed in the multilayer T die, it is difficult to control the respective temperatures of the monolayer sheets individually. Particularly, it is difficult to hold the mid-layer containing a foaming agent at a temperature which will not cause the foaming agent to produce foams until the multilayer sheet is subjected to a compression molding process because the mid-layer is heated by the adjacent layers of molten polymers and the T die.
A cutting process for a sheet formed by extruding molten polymer through T die is disclosed in JPB No.44124/1985. In this cutting process, the sheet is cut to a length by moving a cutting blade in a direction perpendicular to the width of the sheet at the die slot opening of the T die.
When the sheet is cut by such a manner that the molten polymer has a tendency to adhere to part of the T die around the die slot opening, the scorched polymer is liable to adhere to the surface of the sheet, the cutting blade becomes dull due to heating at a high temperature, and the molten polymer is liable to adhere also to the cutting blade during the sheet cutting process.
Because the molten polymer is in a state that has a low viscosity and highly adhesive in a sheet cutting process, difficulty in cutting the sheet by the cutting blade will be enhanced. Therefore, a cleaning means must be disposed near the cutting blade and the cutting blade needs relatively frequent cleaning so that the molten polymer may not adhere to the cutting blade kept on standby near the die slot opening of the T die.
Cleaning of the cutting blade increases the molding cycle time and reduces the efficiency of the manufacturing process greatly. If the cutting blade is not kept satisfactorily clean, the quality of moldings will be deteriorated.
The improvement of the efficiency of processes for manufacturing molded multilayersheet parts of complicated shapes has been a main problem that faces manufacturers. Particularly, in a case that a multilayer article consists of component sheets differing from each other in material, thickness and shape, it is impossible to achieve a series of processes including a extrusion process, a laminating process, a cutting process and a compression process by single reciprocating stroke of the T die.